The trend of recent years in a semiconductor device has been a highly integrated structure, which requires fine interconnects and multi-layered structure. To realize the fine interconnects and the multi-layered structure, it is necessary to planarize a surface of a substrate. Chemical mechanical polishing (CMP) is conventionally used to remove irregularities from the surface of the substrate to thereby planarize the surface.
In the chemical mechanical polishing process, polishing operation has to be stopped at a desired point after the substrate has been polished for a predetermined period of time. For example, it may be desirable to leave an insulating layer, such as SiO2, (such an insulating layer is referred to as an interlevel film because a layer, e.g., a metal layer, is further formed on the insulating layer in a subsequent process) on metal interconnects of Cu or Al. In this case, if the substrate is polished more than required, a surface of a lower-level metal film is exposed. Therefore, the polishing process needs to be finished so as to leave the interlevel film with a predetermined thickness.
In the fabrication process of the semiconductor device, a predetermined pattern of interconnect trenches is formed on a surface of a substrate, and the interconnect trenches are filled up with Cu (copper) or its alloy. Then, unwanted portions of Cu or its alloy are removed from the surface of the substrate by the chemical mechanical polishing (CMP). When the Cu layer is polished by the CMP process, it is necessary to selectively remove the Cu layer from the substrate so as to leave only the Cu layer in the interconnect trenches. Specifically, it is necessary to remove the Cu layer in areas other than the interconnect trenches until the insulating film (which is made from SiO2 or the like) is exposed.
In this case, if the Cu layer in the interconnect trenches is excessively polished off together with the insulating film, a circuit resistance can increase and the entire substrate has to be discarded, resulting in a large loss. On the other hand, if the Cu layer is polished insufficiently and remains on the insulating film, circuits are not separated well and short-circuit occurs. As a result, polishing of the Cu layer should be performed again, resulting in an increased manufacturing cost.
There has been known a polishing state monitoring apparatus for measuring an intensity of a reflected light using an optical sensor and detecting an end point of the CMP process based on the measured intensity of the reflected light. This polishing state monitoring apparatus includes the optical sensor having a light-emitting element and a light-detecting element. Light is applied from the optical sensor to a surface of a substrate during polishing of the surface. An end point of the CMP process is determined from a change in reflection intensity of the light from the surface of the substrate.
The following methods are known for measuring optical characteristics in the above-mentioned CMP process.
(1) Light from a monochromatic light source, such as a semiconductor laser or a light-emitting diode (LED), is applied to the surface, being polished, of the substrate and a change in the intensity of reflected light is detected.
(2) White light is applied to the surface of the substrate, and a spectral (ratio) reflection intensity is compared with a pre-stored spectral (ratio) reflection intensity for a polishing end point.
There has recently been developed a polishing state monitoring apparatus constructed to estimate an initial film thickness of a substrate, apply a laser beam to the substrate, and approximate a time variation of measurements of the intensity of reflected light from the substrate with a sine-wave model function to thereby calculate a film thickness.
There has also been proposed a method of detecting a polishing end point based on a time variation of a characteristic value of a substrate. This characteristic value is calculated by multiplying spectral data, obtained by applying light to the substrate, by a weight function and integrating the resultant spectral data (for example, see Japanese laid-open patent publication No. 2004-154928).
However, in the above-described conventional methods, it is difficult to detect a distinctive point (i.e., a point of distinctive change in the reflection intensity or the characteristic value) which serves as an index indicating a polishing end point. This makes it difficult to detect an accurate polishing end point. For example, when using a monochromatic light source, a relationship between a film thickness and a signal of the reflection intensity is determined uniquely according to a wavelength of the light source. In this case, the distinctive point may not always appear when a target film thickness, i.e., a polishing end point, is reached. Moreover, it is difficult to correct the manner of appearance.
On the other hand, when using a multiwavelength light such as white light, it is possible to select a desired wavelength so that a distinctive point of the reflection intensity appears when a desired film thickness is reached. However, selection of an optimum wavelength for a structure of a workpiece entails trial and error. As a result, a lot of time is needed for the selection process. Moreover, it is difficult to verify whether the wavelength selected is best suited.
A polishing apparatus having a top ring with multiple chambers therein is known as an apparatus for performing the above-mentioned CMP. This type of polishing apparatus is capable of adjusting pressures in the chambers independently. In this polishing apparatus, a sensor is provided so as to measure a physical quantity associated with a thickness of a film on a substrate and a monitoring signal is produced based on this physical quantity. Prior to polishing of the substrate, a reference signal that indicates a relationship between the monitoring signal and times is prepared in advance. During polishing of the substrate, pressing forces of the top ring are adjusted such that monitoring signals, obtained at plural measuring points on the substrate, converge on the reference signal, whereby a uniform film thickness can be realized over the surface of the substrate (for example, see WO 2005/123335).
A highly-functional CPU has recently been developed with the trend of a high-speed and highly-integrated semiconductor device. This highly-functional CPU incorporates therein several functions including a memory section and a calculating section in a single semiconductor chip. In this semiconductor chip, areas with different pattern densities and different structures coexist. Moreover, a chip size has becoming larger year by year, and some types of CCD devices have a film size of 24×36 mm. In semiconductor fabrications, a lot of chips are formed on a single substrate. Therefore, areas with different pattern densities and different structures coexist in a surface of the substrate. Further, for the purpose of evaluating a finished device, a substrate may have an electrical characteristic evaluation pattern that is greatly different from device patterns.
When polishing such a substrate, a change in thickness of a film on a surface of the substrate is monitored by applying light to the surface of the substrate and detecting the reflected light from the substrate by an optical sensor. However, the intensity of the reflected light from the surface of the substrate varies intricately depending not only on the change in film thickness as a result of polishing, but also on the patterns and structures of the devices. Specifically, since a polishing table and a top ring are rotating during polishing, the optical sensor, which is provided in the polishing table, passes through different areas with different pattern densities and different structures every time the sensor scans the surface of the substrate. Consequently, the intensity of the reflected light can vary due to the influence of the device patterns and structures. This varying reflection intensity is superimposed as a noise on a signal indicating a change in the film thickness. In such a case, even if smoothing of the signal is performed, the change in film thickness cannot be accurately monitored because the noise is large. This affects an accuracy of polishing end point detection and a polishing control for a uniform film thickness.
In a case where an object of polishing is a copper film, an eddy current sensor is often used to measure a film thickness. Typically, the copper film is formed by plating. A plating apparatus for performing copper plating generally has cathode electrodes arranged at equal intervals along a periphery of a substrate. A plating solution is supplied to a surface of the substrate, with the plating solution being retained by a seal member. In this state, a voltage is applied between the cathode electrodes and an anode electrode in the plating solution to thereby plate the surface of the substrate with copper. Use of such a plating apparatus can present a problem of variations in film thickness along the periphery of the substrate because of variations in contact resistance of the cathode electrodes or because of sealing performance of the seal member. As a result, the sensor may scan only thick portions or thin portions of the film depending on times during polishing, thus failing to measure an average film thickness.